How many times I’ve been asked “how do I lower the gain of the DHT preamp?” many builders love the DHT sound and yet don’t really need that gain. Of course when you use a transformer load, step down is the obvious choice. And it works really well when you invest in a very good quality OPT.

However, when you use DHT with high rp, generally most of the low current ones, you face a tough challenge with regard to frequency response.

With the hybrid mu-follower topology (aka gyrator) we have the problem that the gain is fixed to nearly mu.

I’m in love with the DHT sound and in a preamp a 01a and the 801a/VT-25 DHT Preamp Update preamp cannot be beaten in my view. They all deliver a gain of 8 with a hybrid mu-follower topology.

This can be overcome in the following way:

We add an attenuator network at the mu output before the source follower circuit. In the above example, I configured the resistor divider R2 and R3 to provide 1/4 gain so the resulting gain is around 2 (6dB). This works like a charm and helps reducing the microphonic noise due to the attenuation of the overall signal. However, the signal headroom is limited but this shouldn’t be a problem as we would expect to run this below 5Vpp.

The rest of the circuit remains the same. The gyrator is the Rev08 and set to about 90-100V depending on the valve, the source follower is configured to operate at 15mA and the cathode SiC bias is using the C3D02060F which are in my experience the best sounding arrangement for this preamp. The filament regulators are the classic Rod Coleman regs.

It can be improved further by adding a DC level shifter to the bottom of R3 (a simple PMOS would work here like shown here) to increase the input DC level of the Source Follower. This will optimise the performance and reduce even further the distortion at higher voltage swing. Of course, this adds more complexity to the circuit.

I will likely hack my 01a preamp as don’t need the gain of 8. A simple gain of 2 would do just fine in my current 300B amplifier.

For more than 6 months I’ve been playing with a lot of satisfaction my 300B amplifier with the 47 driver. Since then, I’ve made 2 modifications:

1. Major one was the introduction of my Open-Baffle (OB) speakers. These are the “Liionidas” based on the LiiAudio C-15 and W-15 drivers. I need to do a proper write-up of these, but have to admit their sound has taken my system to another level. They are simply superb. Detail and dynamics are beyond expectations. Nothing less to be expected from the designer (Leon Huijgen) :

1. Second modification was minor, I removed the Zobel network from the input LL7903 to minimise loading to the source. Also, after several tests I found that the impact in the HF resonant peak is minor as the source has some resistance (below 100Ω) which is beneficial. So the 300B amplifier looks like this now:

I could live forever with this 2-stage system now. It’s capable of driving the OB speakers without an issue. Although for larger rooms you better go with +10W and the 845 in the cooking process is much better suited. I don’t play too loud and the 300B doesn’t let me down at all when playing it loud, I’m in love with it.

Yet, I’m missing the thoriated tungsten sound of the 801a/VT25 or 01a in my system. Recently I posted about this low gain DHT preamp stage based on the 01a, which can be a good option, however introduces another stage.

I’ve been testing on the bench a couple of drivers based on 801a and VT25. They are different valves but not too far in terms of sound in my opinion. Here are the two flavours:

First one is old school design with 1:1 interstage transformer (custom made) and LL7903 in 1:8 SUT to deliver the 35dB of gain I was aiming at. The drawback of this design is the need for a negative supply. However I have already one for the output stage (either 300B or 845) so easy to reuse. The benefit is that the HT supply can be as low as 250V. I’m adding a capacitor multiplier to dial the right anode voltage and provide a low noise HT rail due to the poorer PSR of the transformer loaded driver.

The performance is magnificent. See below test for an RCA 801a delivering 200Vpp at 0.1% THD. The harmonic profile is very nice from H2 to H5:

Now, here is the VT-25 driver based on my previous designs:

The hybrid mu-follower uses a 1kΩ cathode resistor instead of filament bias. This reduces heat dissipation and raw PS voltage requirements, making it easier to implement. Soundwise after my similar implementations with the 46 and 47 isn’t compromised at all. By running it at 15mA and 265V (15V bias) the stage can deliver 200Vpp at low distortion (below 0.15%). The gyrator PCB is the mini version and overall driver physical footprint is quite small. Drawback of this design is the need for a higher HT (400V instead of 250V), however it’s significantly cheaper unless you have a very good IT at hand to reuse.

Both are great options and will be tested side by side. Very likely I will implement the latter one in my 300B amplifier as it’s easier to swap the current 47 driver and introduce this one. Also, can adjust it for the 801a and compare the differences. Good fun!

It was a half-hour job to mod my Mule preamp by swapping out the UX-120 and introducing the 01a (CX-301a in fact), add the resistor divider to deliver low gain on this preamp:

I set the valves at 115V/4mA. The Source Followers are running at 15mA and the resistor divider is actually 100KΩ/33KΩ (Kiwame) which is what I had at hand. It worked like a charm.

The 01a is driving my Slagle AVC into the 300B amplifier. I can say I can hear now the subtlety of the thoriated-tungsten filaments of the 01a with the extra level of gain I was hoping for. The amp sounds very dynamic at high level and with the clarity of the 01a.

This is indeed a clear winner. I will leave this preamp on my system for a while to confirm further impressions. For now, I just simply love it.

For more than 10 years I’ve been experimenting, designing and enjoying DHT pre-amps. Without over-dwelling on this subject, to me DHT preamps bring the unique sound transparency and detail we all seek for. They can make night or day difference in any system. And is not about gain, hence I’m focusing on sharing this blog entry.

I’ve seen too many builders complaining about the gain of the DHT preamps. Well, in many cases you don’t need the extra gain, perhaps a few dBs but not clearly over 15dB which is what you get from a typical DHT of at least mu of 8 and with a hybrid mu-follower configuration which I praise on its sound. Of course, you can implement a step-down transformer (which isn’t easily affordable if you want the best quality) and also rules out easily most of the high anode resistance triodes.

Recently, I posted this preamp version which I’m currently using in my system and am delighted with.

There are few options out there to satisfy each one’s preferences and available valves. The purpose of this entry is to provide a summary of them to guide a few in their quest on the DHT sound.

CX-301a/01a

There are 2 options for doing this pre-amp. I covered the version based on SiC diode bias, however you can do it with a simple cathode resistor:

I will not repeat and say it only once. put the best quality resistor in R5 you can find. That’s my only advice as the impact on sound is significant.

26

The turn of the famous 26 DHT to provide a similar low gain stage. Here is the design:

The 1K2Ω cathode resistor will provide about 6mA when anode voltage is dialled between 140 and 150V. Alternatively you could add a 6-diode SiC array and adjust the bias point. I was asked recently a few times about the 26, and this is how I’d implement it with low gain (about 6dB)

2P29L

The 2P29L is a fantastic little valve. It’s cheap, accessible and provides a great sound and isn’t microphonic. I really love the 2P29L preamp I built and still have it with me. If you want a low-gain version, here it goes:

A 300Ω cathode resistor can be used and the above design will give you same levels of gain as before discussed.

4P1L

I forgot how many version of this preamp which I baptised “Siberian” over 10 years ago. The 4P1L really sings when you run it at 30mA and 150V:

A bit demanding in terms of HT current, but is a worth preamp to build. If you can tame the microphonic noise, it’s a fantastic valve in my view.

VT-25/10Y

Now, if you are a big fan (like me)  of thoriated tungsten filaments and their sound, this valve is unique and sounds like probably no other valve (except 01a). Here is the low gain version:

Again, an array of 6 SiC diodes is possible in replacement of the 300Ω resistor.

CX-371a

If you want simplicity and don’t want to add a source follower, then you need to look at the low-mu valves. The CX-371a (or just 71a) is a perfect valve for this job:

I built one of this back in time and loved it. The above pre-amp is great and will deliver a gain of less than 10dB.

300B

The 300B can play great as pre-amp. Although I implemented in different ways in the past, here is how I prefer it now:

Simply with a 1KΩ cathode resistor, and biased at 30mA you get a very nice sound and extremely low distortion. The gain is just below 12dB.

46

I love the 46 preamp. The 46 (triode-wire) is as great as a 45. I could have included the 45 here as well:

With slightly higher gain at 14dB, this is a great stage.

Needless to say if you want less gain, you can add the attenuator and the source follower like previous designs in this blog post.

27

Wait a minute, 27? It’s not a DHT. Yes, of course. However, it’s a great IHT. I love the globe version with mesh anode. Have a few in my collection and would definitely put it back to use. The timbre and detail is unique.

This is as good as IHT preamp it can get. You can use many different triodes instead, your choice.

Hopefully you find this entry useful. I will add a few more since my list isn’t complete!

I wanted to share a nice review provided by Lionel Borcelot on his journey in building the VT-25/10Y/801a preamp as shared previously. I think this is a good example on what to expect and how to adapt/reuse existing components at hand to get the most out of this design.  Bravo Lionel, excellent job.

“This is the captain’s log in building a prototype 801A/VT62 DAC pre-amp. This was a tandem build with Martial Eisenkremer ( Audio Tube Service) who, as claim to fame, has been requested time and again by Keith Richards to service his amps and guitar before the RS go on stage at Stade de France.  I must mention here that this effort has only occurred as a result of Ale Moglia, Rod Coleman and Thomas Mayer’s pioneering work in DHT research and the very kind attention and help that they provided both in communicating directly and via their blogs which details, step by step, the stages of development together with the schematics. An audio-philanthropic enterprise, no less.

Most of the components in this build are off the workshop shelf. This does not necessarily imply that they are shoddy, just that they had some previous incarnation in some expensive albeit outdated gear. Nevertheless, next build will employ something a bit more luxurious like Lundahls. The original filament transformers were totally inadequate as they seemed incapable of providing sufficient juice. So the next best thing on the shelf was a 100VA toric with 2x 24V secondaries unwound to 16V. Raw DC supply is quite standard, rectified by diodes and provides approx 22V to the Rod Coleman regulators. Not too much care needs to be taken there, the regulators are gyrators and they do a magnificent job. It is very important to read the manual and follow Rod’s step by step set up. Next step will be to implement separate transformers.

I have chosen not to use the heavy metal brigade (LCLC) for the B+, but rather the Bartola gyrator which had done, on previous projects, such a great job on the 26, 45 and 4P1L pre-amps. So B+ stage one is a simple and standard 5R4WGB rectifier circuit, which diode, incidentally, I prefer to all others due to its unfailingly crisp and precise albeit very musical sound. It belies, its appearance, potato masher, and its cheap price. Hiroyasu Kondo used it in his 211 Ongaku.In this scheme, its casual a walk in the park for this rectifier.A simple but effective L (20 mH), C (22uf mundorf) filter, the gyrator is there to do the rest. And it does it magnificently, on top of which you can dial in a very precise B+. All voltages and current are as per Ale Moglia’s schematics (gyrator version), 200V B+, 20mA on the plate.

For the initial firing up and 100 hours burn-in of the tubes the filament is tuned to the datasheet values of 7.5V and 1.25A. This is important in order to give NOS tubes a chance to grow up to their fighting best. I have employed the filament bias route with SIC diodes rather than the clunky and very hot resistors path, it seems, for the curious, to be the elegant way to go, both in terms of space, heat and certainly sound. Very, very clean, crisp, wide and deep soundstage. For those who only know cathode bias this has the distance between Paris and Wisconsin. In these conditions and in spite of the somewhat cheap build, the background is as black as it can get, I mean totally noiseless even with full volume into the speakers. It is a pretty good indicator of a reasonable construction, particularly when the B+ supply is on board, which I do not recommend for a 26 build.

I would like to add here that there a few components that, it is my belief, should not be skimped upon. The grid to ground 47K resistors (if you are not using a volume control there) are silver non-magnetic tantalums, you can really hear them. They are magnificently expensive, another choice is the Charcroft, Z foils. Good quality teflon sockets (HiFI Collective) do not go amiss. Ht wiring is ofc copper silver plated (from Audiophonics). All other connections are made with 0,8 pure silver wire either in teflon sleeving for the filament and coton sleeve for the signal. Last but not least most of the components are mounted in suspension with silent blocks.

I have used a heavy duty (12 mm sides and 5mm top and bottom plates) aluminium case which I do not recommend as there is no ventilation per se. And as you will discover some of the components create a great deal of heat inside the case.. So, I bore several large holes in the bottom plate and installed 5 low noise fans, 2 in adduction, 3 in extraction. They are quite silent and have their own power supply. This allowed me to decrease internal temperature from 51° to 31° max with a minimal delta T to room temperature of 7°. I suppose its ok in winter but with 38° in summer we shall have to switch to the 26 pre-amp.

How does this sound? To my ears like what I had never heard before. The tube is commanding, it can pull out the most minute pieces of information from the dark recesses of the recording, but does not highlight it, it makes it appear where it belongs. The placement is uncanny, everything in its own place, the stage is both wide and deep, everything appears sonically where it ought to be. On good recordings the reality is freaky, prodigious…One did get accustomed to the 26 which shares a great deal with this tube, but there is more of everything with this one. If more is better as your creed this one is for you.

In conclusion, a word about myself and my abominable addiction to music. I am an architect and designer with a 20 year heavy duty addiction to DIY machines, simply because I could never afford the lovely boutique machines that made many pricey promises, but entirely unsure whether they were kept. In 2018 I won a very sought after price awarded by the Liliane Bettencourt Foundation in Paris for the design of an innovative speaker. A 360° degree emitting dipole, 100Hz to 18Khz. Please stay tuned… »

Lionel Bourcelot

kerienpartners@gmail.com

kerien.com

I’ve received several emails from people asking me why I’ve gone quite lately. Nothing to be alarmed, I’m just simply buried on the daily grind like many out there are on these Pandemic times. My work-life balance has changed dramatically. This left me with very little time on my hands to work on audio stuff. I won’t complain as many others are actually struggling to survive, that is tough.

As my system is playing amazingly well and I’m so satisfied with my 300B-Liionidas OB setup and the low-gain 01a preamp, I do spend every single time at nighttime to enjoy music. That is what is built for in the end, isn’t it’?

Having some time to update on a long-time design, I received last week the remaining iron needed for my 845/813 SE amp. Yes, the monster one. If you see the output transformers that Alexander built for me, you will say the same. They are 40cm tall and 10.5 Kg weight each! Massive, but needed for a  good performance. I originally intended to run these at low-ish operating point, like my friend DHTRob does. With a Zaa of 5KΩ you can do 10-11W with 700V at the anode.  Otherwise you can increase the anode voltage to the crazy 1kV or beyond to get much more power, if you really need that.

In my case, I want to aim for a sensible HT and enough power to drive well my OBs.  You may have other needs and there are so many ways to skin a cat so I’m not going to open up the debate which is pointless.

If you’re interested, here are the specifications of the OPTs:

Impedance ratio:                               4.97k/4Ω

4.42k/8Ω
4.97k/16Ω


Primary inductance:                          75H

Nominal primary Idc:                       120mA

Max power output at 25Hz*:           35W

Primary coil Rdc:                              130Ω
Secondary coil Rdc 16Ω:                   0.48Ω

Secondary coil Rdc 8Ω:                     0.27Ω

Secondary coil Rdc 4Ω:                     0.12Ω

 

DC flux density at 120mA Idc:         1.12T  

AC flux density at 420Vrms**:         0.47T




*Theoretical power output without the losses included

**This primary AC voltage is equal to the maximum theoretical power output

Shunt capacitance:                            1.55nF
Leakage inductance:                         13.2mH
Cp/Ls resonant frequency:              100kHz

Cs/Ls resonant frequency:               54kHz

Frequency response (1700R Zout ; -3dB):

4Ω                                                       3Hz-54kHz

9Ω                                                      3Hz-52kHz
16Ω                                                      3Hz-55kHz

Some driver experiments

I’m looking to build this amp around the 801a valve. The topology is very similar to what I’ve been using lately. A SUT 1:8 with the LL7903 and the custom IT 1:1 directly into the 845 or 813 grid in fixed bias. Simple and yet elegant:

I get 34.5dB gain here and with a very low distortion. See the 250Vpp swing at 0.1%. That’s brilliant with nice harmonic decay:

The alternative option I will test as well, is of course the board I have to complete to replace my 47 driver in the 300B. It’s the hybrid mu-follower version:

Here is with the VT-25, but can be tweaked to run the 801a in the same way. Only challenge is wiring carefully the regulator to avoid hum being picked up as the Rod regulator is high impedance and we have a 1K (un-bypassed) cathode resistor. If you fail to get this working well you have 2 choices, you either add a bypass cap or you use a different regulator (e.g. voltage regulator) which has low output impedance.

Meanwhile, I’m waiting for the superb sockets from Jakeband Audio.

Hope you are all keeping well and safe…

Ale

After a long time, decided to update the PCB of the LL1943 SUT to provide extra flexibility on the grounding connection. This is in essence to ensure no ground loops and noise when either using balanced or un-balanced cabling from the cartridge into the PCB:

The additional jumpers are in line with the recommendations from the Datasheet. Here is the diagram:

For a fixed connection (like in most of the cases once preamp has been tuned/optimised) you can replace the jumpers by solid core wire jumpers soldered to the pad for best connection and performance.

I made a small batch (I won’t need more than 2 or 4! myself) so the remaining will go on first-come-first-serve basis.

As several DIYers out there, I’m a big fan of the Russian teflon FT-3 capacitors in my preamps. I normally use the 220nF/600V ones as have plenty of them in stock. These are superb in my view, however mounting them is a royal pain in the back. I used turrets for some time, but I ended up building a simple PCB which has a cutout to fit the FT3, solder it firmly on big copper pads and then solder the connections from the PCB. Also added a bias resistor (either for charging/discharging the cap or for fixed bias) which is connected to a GND terminal. This GND terminal could be used for the negative fixed bias if required. The PCB can be stacked very easily as has 4 M3 hex standoffs:

Here is a very nice build from Dan Kercher on his contraption of the 300B amplifier with auto bias. The driver is an EF86 with SiC diode biasing, the topology is a hybrid mu-follower and a source follower drives the output stage. Filaments are DC with Rod Coleman’s board:

More details can be found below:

Hi Ale, 

I wanted to thank you for all of your PCBs.  I just finished a project that uses your Gyrators, SiC bias boards, and SiC Source Followers.  I'm so happy with the results.  Details posted on Audio Asylum.  

https://www.audioasylum.com/forums/tubediy/messages/27/276814.html


Cheers, 

Dan 

A few years back I went down the path of exploring different DHT in pre-amp stages for sonic improvements on detail and dynamics. I explored all sorts of different DHTs as have posted in the blog and many more I never got around to write up about.

An interesting brick-wall was hit when testing DHTs with cathode degeneration resistors of considerable size (e.g. higher than what normally is used under filament bias topology) which puts at its limit the traditional filament circuits which are based on CCSs and do want to see a low impedance to GND.

When working with my beloved RE084, I found this issue. Tried the Rod Coleman and Tentlab regulators without success. Below is an extract of my e-mail exchanges with Rod Coleman and Guido Tent.

The issue

Back in 2018, I built the below pre-amp. This needed a 470Ω degeneration cathode resistor:

Now, if you remove the CCS filament regulators and replace them with a low impedance ones (e.g. a bench DC power supply), things look very different.

You can see that the distortion is down to 0.03% for 20Vpp which is improved thanks to the degeneration of the cathode resistor.

Scratching my head, I switched back to the old filament bias, which obviously worked fine:

You can see below, the response is really good (LF and HF). If you remove the Source Follower still is good up to 100kHz or more. When loaded with 100K the LF pole moves up obviously to around 10Hz, which is what is expected.

The distortion profile was really good, below 0.01% for 10Vrms, great performance. It actually sounded very very nice indeed!

So, it was a clear issue on the regulators. I moved back to try Tentlabs modules which are very good. To my surprise, result was the same.

So, had to rig up a test setup on the bench to measure noise the best I could. Here is what I’ve got:

I fed this back to both Rod and Guido back into 2018. Cleary, I pushed their regulators into the unexplored territory. Fine, they work brilliantly in other topologies, but not on this one. Obviously for more traditional designs (Choke, resistor or transformer load) you wouldn’t hit this issue as you wouldn’t look to implement degenerated cathode bias with a high Ra triode! Only it’s possible under the mu-follower topology, which I obviously have drained out like not many out there – BTW: this was JC Morrison’s comment at the last ETF!

There is some light at the end of the tunnel

[..]
I'm returning to this email from 3 years ago (my apologies for the slow work!) - because I think I have found out what is going on with Low-current filaments and my regulator. 

I was looking at some nice old Tungsram (Tottenham factory) LP2s with a 200mA current filament, and built up a 150V stage to test them. That's when I confirmed the excessive spur at 50Hz in the spectrum.

I started by measuring the same circuit that you used to test the RE084 with Guido's regulators, and compared to mine, and your voltage regulator (the schematic and noise spectrum attached to your message of 31 March 2018). 

with my V8 regulator, the 50Hz spur was at ca. 100µV - just the same as you measured. Guido's measured the same - because it has the same problems. It turns out that using a current regulator in the negative line means that you have weak coupling between the Raw DC negative, and the system ground, that allows noise to get coupled in from the transformer, or from the circuit wiring, and it has no means of suppressing it. The noise then gets pushed through the positive side of the filament. This also causes higher distortion, since the filament is bouncing around with any noise it picks up...

Identifying the suspect quickly suggests a solution! So I re-designed the regulator to allow the negative side to have low impedance, and the current regulator moves to the positive side. At the same time I added some nice upgrades that I found in the development of the Transmitter regulators.

Of course, this is the same polarity that your voltage regulator has - hence no noise problems. 

It's not the differential impedance of the regulator that matters - it's the ground path.

With this new V9 prototype, the new regulator measured 40dB better, or ca. 300nV at 50Hz (instead of 30-100µV for V8, or Tentlabs  - it can get worse, depending on where the safety earth is connected).

The attached spectrum shows the new results - between V8 and V9, connected to a LP2 filament, and 27R bias resistor. The spectra are calibrated for 0dB = 700µV - it's a high-gain Mic amp. Same circuit as your RE084 fil. bias testing.

The V9 is sounding better than the V8 - because of the circuit improvements, and solving this problem no doubt adds even more to it. 

With filament bias it will be quite an improvement in the sound, since even without hum, there is some corruption of the anode current.

I am hoping it will also fix the problem with unbypassed cathode resistors - but I have not done full tests yet. [...]

Rod Coleman / August 2021

I added these PCB for sale. I made a small batch for my own use (you can see them in the 300B amplifier here) but am selling the rest of them.

I took me a couple of years but eventually found the time to build the BOM and a simple documentation guide for the DHT LT supply PCB. I’ve been using this PCB for a long time on most of my DHT preamps and in my current 300B SE Amplifier.

It’s a 16 x 8 cm board which can be stacked for a stereo DHT stage. Enough flexible is provided for all sort of filtering combinations (either choke to cap input supplies).

Some may say is a killer, but I add a common-mode choke to suppress HF noise from mains. I design my supplies with a no-compromise thinking and best possible components (e.g. including split-bobbin main transformers), chokes and capacitors to deliver the lowest noise possible to the filament supply.

This board doesn’t replace the filament regulator (e.g. Rod Coleman regulator), it’s just the raw supply circuit needed before them.

I will sell the remainder of my stock of these lovely ENIG finished PCBs. You can order them here.

It’s been a long since I last posted here some project stuff. I’ve been busy with work and family. On my spare time, spent mostly listening to music, developing some synthesiser modules and playing music as well!

I missed ETF this year, sad to say. Work commitments made it impossible to  travel to France. Hope that’s not the case next year as am keen to get back. I’ve seen some pictures and feedback and seems it was a blast despite the COVID restrictions.

On my exchange with Rod Coleman regarding this post, I asked him to send me a pair of his latest DHT filament regulator (version 9) to test them on my 801a preamp with a cathode bias resistor (degenerated or un-bypassed):

These regulators are design with the objective in mind of having a larger impedance path in the cathode. The negative terminal in this version always need to connect to the cathode resistor.

I will modify the VT-25a/801a preamp I built back in 2016 to use these regs. I will need to remove the filament bias element and replace with the cathode resistors. I will also upgrade the gyrator boards and get these boards for the FT-3 caps I have in place:

Some new holes will have to be drilled I think on this nice aluminium top plate. Let’s see how it turns out.

The circuit has already been presented:

I won’t be adding initially the low gain attenuator network (R2 and R3) nor the source follower. The rest is the same as above

It’s always satisfying to see someone else enjoying the end result of their build work. Here’s a great example of Nash building his take on the 01a low gain DHT pre-amp:

Hi Ale,

Its now over two months since I have been listening to your low gain version of the 01A preamp in concert with a Slagle AVC after it.

Over a span of a few years I have built several preamps and I can honestly say that this one is substantially better than any of the others. As a classical music fan I value attributes like clarity, timbre, body, weight and speed and your design delivers! I would like to use the analogy of a superb cake with icing- the cake is the 01A pre and the icing is the AVC. Thank you for a great design!

I have recently replaced RMu and the divider resistors with the TT Electronics PFC series SMD resistors mounted on a 1206 adapter. These are tantalum nitride resistors and they sound wonderful in this application. 

Further, I am getting interested in your 300B SE builds. Are you using a preamp before these? Please send me a link to your latest work in this area which you feel I should consider.

Regards,
Nash Bapasola  in NJ

About 5 years ago I built the first version of the Aa DHT preamp. It looked like this:

This preamp sounded great (as you would expect with this DHT valve). As wrote before, with a gain of about 30 is ideal if you need the gain. More importantly, it’s timbre and detail is unique but is challenging to use. It has an internal anode resistance of about 30KΩ, so best topology is what I presented before as choke loaded is a real challenge.

The filament bias configuration with the Russian wire wound resistor of 5Ω is perfect drop-in. Later I tried the SiC bias configuration which was also great.

Recently I was asked for the Spice model. I thought I had posted it before, but I was wrong. So here it is:

I traced the curves of a Valvo Aa valve which tested 97% (Ia=2.92mA/1.09mS @ 220V/Vgk=-2V):

And here is the manual fit of the DHT model:

Below is the model, and you can also download it from here: Aa spice model:

**** Aa TRIODE Composite DHT *****************************************
* Created on 10/13/2017 18:31 using paint_kit.jar 2.9 
* www.dmitrynizh.com/tubeparams_image.htm
*
* Traced and model by Ale Moglia valves@bartola.co.uk
* (c) 2017 Ale Moglia and Bartola Ltd. UK
* www.bartola.co.uk/valves
*----------------------------------------------------------------------------------
.SUBCKT DHT_Aa 1 2 3 4 ; P G K1 K2
+ PARAMS: CCG=1P CGP=3.5P CCP=1P RFIL=7.6
+ MU=30.3 KG1=4740 KP=216 KVB=720 VCT=-1.33 EX=1.41 RGI=2000
* Vp_MAX=350 Ip_MAX=8 Vg_step=0.5 Vg_start=0 Vg_count=11
* Rp=4000 Vg_ac=55 P_max=1.5 Vg_qui=-48 Vp_qui=300
* X_MIN=69 Y_MIN=51 X_SIZE=437 Y_SIZE=429 FSZ_X=1134 FSZ_Y=689 XYGrid=false
* showLoadLine=n showIp=y isDHT=y isPP=n isAsymPP=n showDissipLimit=y 
* showIg1=n gridLevel2=n isInputSnapped=n 
* XYProjections=n harmonicPlot=n harmonics=y
*----------------------------------------------------------------------------------
RFIL_LEFT 3 31 {RFIL/4}
RFIL_RIGHT 4 41 {RFIL/4}
RFIL_MIDDLE1 31 34 {RFIL/4}
RFIL_MIDDLE2 34 41 {RFIL/4}
E11 32 0 VALUE={V(1,31)/KP*LOG(1+EXP(KP*(1/MU+V(2,31)/SQRT(KVB+V(1,31)*V(1,31)))))} 
E12 42 0 VALUE={V(1,41)/KP*LOG(1+EXP(KP*(1/MU+V(2,41)/SQRT(KVB+V(1,41)*V(1,41)))))} 
RE11 32 0 1G 
RE12 42 0 1G 
G11 1 31 VALUE={(PWR(V(32),EX)+PWRS(V(32),EX))/(2*KG1)} 
G12 1 41 VALUE={(PWR(V(42),EX)+PWRS(V(42),EX))/(2*KG1)} 
RCP1 1 34 1G 
C1 2 34 {CCG} ; CATHODE-GRID 
C2 2 1 {CGP} ; GRID=PLATE 
C3 1 34 {CCP} ; CATHODE-PLATE 
D3 5 3 DX ; FOR GRID CURRENT 
D4 6 4 DX ; FOR GRID CURRENT 
RG1 2 5 {2*RGI} ; FOR GRID CURRENT 
RG2 2 6 {2*RGI} ; FOR GRID CURRENT 
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N) 
.ENDS 
*$

About 10 years ago I used this tetrode driver which I loved. The 24. It has hungry filaments, albeit a lovely sound. I probably lost a few readers by now, but don’t care. The 24 into the 45 Single ended, was a great choice, no wonder why Torsten praised this valve.

I settled with the following driver which worked well for me. The 24 is carefully operated at 8mA and 80V on the screen to minimise distortion. This way you can get about 35.5dB (x60) gain stage with about 160Vpp output at 0.5% THD. Yes, more H3 component than a triode, but sounds really nice:

Simple and effective gain stage for a 2-stage amplifier. The output valve could be anything of your choice: 2A3, 45 or even 300B.

The hybrid mu-follower use R1 to set the gain of the tetrode stage (I covered this before many times) and the stable mu-output also feed the screen regulator (Reg 1). I made also a PCB for this regulator, so the driver footprint is very small.

Long time ago I breadboarded the 1624 valve and liked the sound.  The 807 is famous and I never ended up with a permanent system with this valve. Yet, I like it. And if you have them at hand, use them. Even for guitar amps are amazing.

This is what I used. A 1:4 SUT with  the LL7903 driving a PMOS – any of your choice will do. The  output would look something on these lines:

Which was very pleasing to my ears as far as I could remember. Now, look at the driving signals:

You can see that the grid will swing about 20V. but the source of the MOSFET will go from nearly -180V to -40V so about 140-150V swing.

I never tried this, but if you’re seeking for lowest distortion profile, you can implement this, not cheap but possible:

The depletion FET (IXCP10M90S in simulation but can be anything you like that can stand 1kV or so) needs a 700V source voltage to allow full swing of anode voltage albeit not loading the anode with the feedback loop. Clever arrangement but expensive, or audiophile-stupid level proof.

If you do this, you would improve the distortion of this stage to this:

And yes down from 1.1% to 0.35% for 4W output. Worth looking into.

Happy Sunday.

Goodbye Jon

Sadly, yesterday heard the sad news that our friend Jon Finlayson from the London Audio Circle passed away.  Jon was an incredible person, a true music lover and passionate about life. He introduced me to most of the circle team members and enjoyed thoroughly the meeting at his place and listen to his electrostatic speakers and constantly evolving system.

I learnt such a great deal of stuff from him and will remember our conversations and how passionate and meticulous he was into doing things. In particular when I built my Starlight CD player and he helped me out with the DAC ladder resistor matching.

See you on the other side Jon.

The 811a amplifier

The last year of the pandemic allowed me to do some experiments whilst being more at home. I hooked up an 811a on the bench with the following circuit:

I reused my original D3a driver from the 300B SE Amplifier. That was an easy choice and speed up my building process. I added a PMOS driver for a “plate to grid” feedback. The feedback network was somehow tricky to trim as had limited resistors available but the combination of 2 39KΩ resistors and a 18KΩ resistor worked well. The follower helped with driving the positive grid current which is about 16mA on idle at 22.5V bias or so. The bias is actually adjusted with a positive regulator set to about 50V on the PMOS gate.

I ended up building a PCB for the PMOS driver, very simple but effective.  I used an SMPS for the filament supply and was good enough for testing purposes.

The iron was some Lundahl gapped at 90mA which I had at hand, but can be anything you want.

Power? Oh yes, could extract 16W at about 1.5% and think measured 18W before clipping.  Distortion at low power (e.g. 2W) was less than 0.4% so a nice beauty to listen to. I love the sound I got out of it, not my normal listening system and couldn’t move the whole Frankenstein down to where my speakers are with the kids around and 600V all over it (long are gone those days where 600V were everywhere in my music cabinet).

Brief post of a very long experiment. Unfortunately I’d have to dig out a long list of notes, tests and simulations to get the full thing out there. Unfortunately due to constraints, this will be brief but I hope it’d be entertaining.

The design is straightforward. JC Morrison has also worked with similar screen feedback arrangements but not with a hybrid mu-follower stage. The feedback loop is taken from the mu-output in my case. I’m using the EF37a with slight positive G3 bias (+12V) to minimise distortion at largest voltage swings (200Vpp). The gNFB is anode of output stage into cathode of the driver via R4 and R8. There are 2 tricky LF poles playing with the screen bias (C5+R9+divider pot) and C9+R14 in addition of the HF pass made up of C6 and R4.

I had some LF oscillation in my breadboard which I couldn’t fix eventually despite the help from JC and a few iterations.

The 6P36S is stressed to its limits. This valve is famous for being able to deliver much more than the specs. Yet, it was (not sure if still is) cheap as chips and I have plenty of them. I needed +5W, and here is the test with 7W:

Not bad at all 0.3% and decent harmonic profile.

I’d probably go with a different feedback arrangement if I were to do this again in future. Cathode drive is more interesting in my view and there are PMOS around to do this:

You will need a proper sized heatsink for the PMOS as it will dissipate 11W and R3 will help out a bit with 2.5W at least. 160Vpp are needed to drive the cathode so many of the good drivers are out there.

It’s been long since my last post. Life is busy for me these days, so not much time free for Audio work. I managed to rebuild my listening room after nearly a year of disruptive and delayed building works. So am a happy man on that front.

I hope I can upload some work soon. In the meantime, my friend Geoff raised to me this particular scam, unbelievable:

https://archivebooks.online/download/4677785-bartola-valves-all-about-electronic-valves-and-hi-fi

Amazing to see that there are people out there trying to get your details with this. I can’t even imagine what they are expecting you to do after registering in order to download allegedly some “book” which I never wrote or commissioned.

Anyhow, you’ve been warned. Don’t even it